1. Field of the Invention
The present invention relates generally to an oscillator operating with a variable driving voltage, and more particularly to an oscillator operating with a variable driving voltage capable of varying the driving voltage of the oscillator in order to regulate the oscillation signal period produced by the oscillator.
2. Description of the Prior Art
In general, an oscillator produces a pulse signal having a predetermined period, and FIG. 1 is a simplified circuit showing the basic structure of a conventional ring oscillator.
A conventional ring oscillator as shown in FIG. 1 includes six inverters and one NAND gate. The inverters are connected to each other in the form of a ring. Each dotted box represents a capacitor connected to or separated from a node of the ring by, for example, a metal option.
Referring to FIG. 1, the conventional ring oscillator is controlled by an enable signal Enable, and the oscillator outputs an oscillation signal of a predetermined period while the enable signal is maintained at a high level. In general, the period of the oscillation signal outputted from a ring oscillator is influenced by the number of the capacitors connected to the ring (as shown in FIG. 1 by metal option). For example, if the number of capacitors connected to the ring increases, an RC delay time will increase, and this will produce an oscillation signal having a longer period. However, if the number of capacitors connected to the ring decreases, the RC delay time will be shortened, and the period of the oscillation signal will also be shortened.
Additionally, the oscillation signal period is influenced by variation in the manufacturing process of the oscillator and the voltage and temperature conditions of the oscillator in operation, and for these reasons, a designer generally provides extra capacitors to the circuit, which are available to be connected to the ring of an oscillator to adjust the period of the oscillation signal. These extra capacitors can be connected to the oscillator or separated from the oscillator by a FIB device (Focused Ion Beam device).
Accordingly, the designers can regulate the oscillation period of the oscillation signal by controlling the number of the capacitors connected to the oscillator (e.g., to the nodes of the ring) by using the FIB device. The work to optimize the oscillation period by connecting capacitors to the oscillator or disconnecting the capacitors from the oscillator by using the FIB device is ordinarily performed at an early development stage of the semiconductor devices.
However, such a work to connect and disconnect the capacitors causes the test time and the costs to significantly increase, especially when a large number of oscillators exists in a semiconductor chip.
Additionally, the extra capacitors provided in a ring oscillator lead to increased ring oscillator circuit area in a semiconductor chip and this imposes a serious burden on the designer when a large number of oscillators is used in a semiconductor chip.